Network Working Group J. Peterson
Internet-Draft NeuStar
Intended status: Best Current November 8, 2007
Practice
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Normative Language and References
draft-peterson-informational-normativity-01
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Abstract
This document explores the use of normative language and references
with a focus on reducing unnecessary normative references in IETF
specifications.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. What is 'normative'? . . . . . . . . . . . . . . . . . . . . . 4
2.1. Normative Pseudo-Keywords . . . . . . . . . . . . . . . . 5
3. Normative references . . . . . . . . . . . . . . . . . . . . . 6
3.1. Examples and Ambiguities . . . . . . . . . . . . . . . . . 8
4. Normative Language off the (Standards) Track . . . . . . . . . 9
4.1. Informational Publication of Protocols . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
8. Informational References . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
RFC2119 [1] provides a set of familiar directives to readers of IETF
specifications, specifically the imperatives: "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL". This set of normative
keywords, as they shall be known in this document, consists of a
number of grammatical variations which ultimately describe three
degrees of normative compliance: mandatory, recommended, and
optional. The first two degrees may be used in either prescriptive
or proscriptive contexts (e.g. "MUST" and "MUST NOT", "SHOULD" and
"SHOULD NOT"), while for the third only prescriptive statements are
permitted (there is a "MAY" but no "MAY NOT", something can be
"OPTIONAL", but not "NOT OPTIONAL").
The use of normative keywords is one of the defining characteristics
of IETF specifications. Normative keywords remain an indispensable
tool for evaluating interoperability as specifications advance on the
standards track, and moreover for pruning unimplemented features as
protocols mature through deployment and usage. The application of
normative keywords to these functions is predicated largely on the
text of RFC2026 [2].
RFC2119 does not, however, contain the word 'normative', and nor does
RFC2026. The idea that a statement or reference can be 'normative'
or 'informational' (let alone the requirement that the References
section of an Internet-Draft be divided between the two) dates from a
much later time, as does the term 'normative language'. The
conditions that render a particular reference or statement
'normative' have never been specified; although there is a good
understanding in the community of the common distinctions, practices
can be very erratic in corner-cases.
An example of the resulting confusion is the use of normative
keywords in requirements documents, which here are to be understood
as Informational documents that apply constraints to future protocol
specification work, as opposed to actual implementation work.
Authors of standards-track protocol specifications intended to
satisfy these requirements sometimes include such requirements
documents in their "Normative References" sections, precisely because
they are referring to statements containing normative keywords. This
sort of downward reference is of course formally prohibited in
RFC2026, and thus must corrected, but the whole situation arises
needlessly. In the absence of some clarification, similar
misconceptions will continue to arise.
This document therefore attempts to provide a stronger account of the
classification designated by the term 'normative', and to detail
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various conditions under which a reference need not be considered
'normative'. RFC3967 [3] wisely notes that normative references
resist definition, and the text in this memo does not claim to have
articulated all of the associated subtleties and implications;
however, in order to reduce overuse and misapplication of normative
terms, a more substantive account of the term 'normative' appears
here than has in RFC3967 or previous works.
RFC3967 and more recently RFC4897 [4] have revised the guidance of
RFC2026 regarding the advancement of standards-track documents which
refer to documents at a lower maturity level (or those not on the
standards track at all). The present document is entirely compatible
with the useful amendments introduced in those documents.
2. What is 'normative'?
Normative keywords are 'normative' in so far as they establish the
norms that are the foundation of interoperability. Implementations
of a particular specification can be considered to be a sort of
community, and that community has practices that are mandatory and
prohibited, recommended and counterrecommended, or simply optional -
hence, they are norms.
'Normative language' or 'normative statements' are, broadly, passages
of text in IETF documents which contain normative keywords that
direct implementers, with varying degrees of stringency, to
incorporate particular features in order to foster interoperability.
Normative language, as originally described in RFC2119, is tooled
solely to describe how implementations are intended to behave. As
RFC2119 Section 6 states, in reference to normative keywords:
In particular, they MUST only be used where it is
actually required for interoperation or to limit behavior which has
potential for causing harm (e.g., limiting retransmissions)
Ironically, this normative statement is not internally consistent.
It urges authors of specifications to use normative keywords only in
reference to matters of implementation, but in order to amplify its
point from mere urging to absolute dictum, it relies on a normative
keyword. Therein lies the source of the confusion. Normative
keywords are used commonly, but incorrectly, in precisely this
fashion: for emphasis, in passages of descriptive text that in no way
could be construed to address implementations.
When authors of subsequent specifications see such normative keywords
used in an purely descriptive passage in an RFC, they may assume that
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the document containing those normative keywords should be referenced
normatively. This can cause an unnecessary apparent need for a
downward reference.
Any statement that is non-normative is by definition purely
informational. Informational or descriptive statements play a large
role in IETF documents, providing context that is useful to
implementers or authors of future specifications but which does not,
strictly speaking, detail implementation behavior that will
subsequently be measured for compliance or interoperability.
2.1. Normative Pseudo-Keywords
The use of the normative keywords has never been compulsory in the
IETF. Numerous documents, both before and after the publication of
RFC2119, describe protocol behavior without relying on normative
keywords. Normative keywords are a way of explicitly designating the
degree of compliance associated with a behavioral prescription for
implementations. It is equally possible, and sometimes more
readable, to write precise text which is semantically identical to
passages employing normative keywords. Constructions with that
property are herein said to contain "normative pseudo-keywords":
prescriptive behavioral keywords that could potentially be
paraphrased with normative keywords.
In order for such a piece of text to qualify as normative, it must
contain pseudo-keyword text which designates the degree of compliance
levied on the behavior (i.e. terms that recognizably signify
'mandatory', 'recommended', or 'optional)'. Furthermore, following
the definition of the RFC2119 keywords, these statements must pertain
to implementation behavior. Statements can be tested by paraphrase;
for example:
Messages are challenged using Digest authentication [RFC2617].
The passage above uses a normative pseudo-keyword (in this case, the
verb 'to be') which can be paraphrased using a normative keyword as
follows:
Messages MUST be challenged using Digest authentication [RFC2617].
However, it is equally possible to generate behavioral statements
which do not satisfy this test, for example:
Messages might be challenged using Digest authentication [RFC2617]
when a weaker form of authentication would be inappropriate.
Is this a recommendation, a counterrecommendation, or what? Without
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sufficient supporting text to suggest when weaker forms of
authentication might be appropriate, one could even read this as a
mandate for the use of Digest. In the absence of a recognizable
degree of compliance, this statement cannot be normative. In the
interests of promoting interoperability it would ideally be replaced
with a clearer sentence containing a normative keyword.
The use of pseudo-keywords to form normative statements is never
completely unambiguous, and is therefore discouraged. It is not at
all uncommon today for reviewers, at the IESG review phase or
earlier, to provide blocking comments on Internet-Drafts of the form
"is this 'must' supposed to be a 'MUST'?" The use of normative
keywords, since they have accepted definitions within the community,
are the most precise way of designating a degree of compliance. That
much said, authors of specifications must also recognize that
normative keywords are not a panacea, and that gibberish can be
written just as easily with uppercase words as with lowercase. Some
examples of this are given in Section 3.1.
However, for the purposes of normative references and the evaluation
of features and options, passages containing normative pseudo-
keywords are treated as equivalent to passages containing normative
keywords. Without this allowance, it would be impossible for new
documents to refer normatively to many, if not most, existing RFCs.
It is the semantics, not the syntax, of statements that is crucial to
determining their normative status.
3. Normative references
This document follows the terminology of RFC4897 for a 'source
document' (a document in which the reference to another document in
embedded) and a 'target document' (the document so referenced). It
furthermore defines the 'referencing statement' as the statement in
the source document which invokes the reference to the target
document. A 'normative statement' is understood to be a statement
which uses normative keywords (or pseudo-keywords) to associate a
specific degree of normative compliance with a particular
implementation behavior.
For the benefit of specification authors, the following is a list of
conditions in which a reference to a document need not be normative:
1. if the source document is itself Informational (not a standards-
track document, BCP or an Experimental document).
2. if the referencing statement is not a normative statement; i.e.,
does not prescribes some degree of normative compliance with the
target document.
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3. if the target document, and in particular any subset scope
designated by the referencing statement (a section, or what have
you), contains no normative statements.
If any of the above conditions apply, then the reference in question
need not be normative. One additional possible condition, that the
target document have an equal or greater standards maturity level to
the source document, is not strictly speaking a necessary condition
for a normative reference; however, normative references made when
this condition prevails must successfully invoke the downref
exception procedures defined in RFC3976 in order to advance on the
IETF standards-track.
One source of ambiguity in determining whether or not a reference is
normative is the status of Best Current Practices (BCPs, as defined
in Section 5 of RFC2026). The BCP designation is a bit of a catch-
all in the IETF standards process. A BCP can prescribe practices
varying from operations, which are indeed critical to the
interoperability of the Internet, to IETF process, which is of a non-
technical nature. As such, it is entirely appropriate, in some
cases, to provide a normative reference to a BCP, and for a BCP to
contain normative keywords.
In the case of IETF process BCPs, it is less clear that they should
be understood normatively, and moreover less clear that it is
appropriate for process documents to employ normative keywords. A
precedent for using normative language in those documents was set by
RFC2418 [5] (see especially the last paragraph of Section 1; this is
also discussed further below in Section 4). When process documents
do employ normative keywords, as RFC2119 does in the citation above,
it is almost always inconsistent with the definition of those terms
in RFC2119 and their intended use in RFC2026. This in turn further
contributes to the perception that it is appropriate for non-
technical documents in general (such as requirements documents) to
employ normative keywords. Unfortunately, this appropriateness of
using normative language in BCPs must be assessed on a case-by-case
basis.
A good rule of thumb, and a corollary of the guidance in RFC3967, for
whether a reference should be normative or not is the following: if
the target document were lost, such that an implementer could not
read, access or implement it, would implementations of the source
document still interoperate for the functionality described in the
statement? If not, then the reference must be normative.
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3.1. Examples and Ambiguities
Even so strict an account of a normative reference cannot be entirely
free from ambiguities that are grounded in the degrees of compliance
themselves, especially regarding conditionals and conditional
prohibitions (e.g. "SHOULD NOT"). Ambiguities also arise when
degrees of compliance are associated with the use of a feature rather
than its implementation. Consider the following referencing
statement:
Implementations SHOULD NOT use 3DES [1].
If this had said that implementations "MUST NOT" use 3DES, it would
be unambiguous that implementations were not required to implement
3DES, and thus the reference to 3DES would not be normative. You
would never need to implement 3DES in order to be able to
interoperate with another implementation of this specification. But
what about the conditional prohibition "SHOULD NOT use 3DES"? Is it
necessary to support a referenced specification in order to obey a
conditional prohibition against using it? This is difficult to
answer on its own, but in fact, that referencing statement entails
this one:
Implementations MAY use 3DES [1].
Should 3DES be considered a Normative or Informational reference if
it is OPTIONAL? Normative, clearly, by the litmus test. Both this
statement and the one above allow, but do not require,
implementations to support 3DES. And because this statement is
entailed by the conditional prohibition above, both of these are
making a normative reference to 3DES. This may appear to be
something of a paradox, since it seems intuitively that
counterrecommended behavior shouldn't require a normative reference,
but optional behavior should - one just needs to bear in mind that
everything counterrecommended is necessarily optional.
Given all that, better specmanship would make the exact
implementation needs more clear. A closely analogous but infinitely
superior phrasing would be:
While 3DES [1] is unsuitable for use in most environments,
for backwards compatibility reasons implementations MUST
support it.
In this case it is clear that support for the 3DES protocol is
mandatory, even though its use is discouraged (non-normatively).
There are also classes of statements that employ normative keywords,
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and contains references, but do so in a way that does not actual form
a normative referencing statement. Consider this:
The application MUST implement an underlying transport which
can provide integrity and confidentiality properties, for
example TLS [3].
The citation of TLS above is merely exemplary; the referencing
statement does not actually require application developers to
implement TLS. Rather, it requires that any underlying transport
that is implemented have certain properties, though not terribly
specific ones. As such, this reference cannot be considered
normative - it suggests no specific underlying transport to the
implementation community. Precisely for this reason, it is an
example of weak specmanship. Statements of this general form often
seem attractive, however, to specification authors who hope to
reference work-in-progress or Informational documents. The fix for
this sort of specmanship is not to require TLS to appear in the
Normative references section of the document (it shouldn't on the
strength of this statement), but rather to encourage the authors to
make a stronger referencing statement, one actually conducive to
establishing implementation norms.
Another similar example is the use of disjunctive references like the
following:
Implementations MUST provide this capability by implementing
either PGP [4] or S/MIME [6].
Is this a normative reference to both PGP and S/MIME, or neither? It
seems to read that implementers would only need to implement one in
order to be compliant, so perhaps only one of them is actually a
normative reference... but if so, which one? Ultimately, this is
another instance of weak specmanship, in which the statement itself
needs to be amended before its clear what the normative requirements
are.
These examples are hardly exhaustive, but they at least serve to
motivate the need for a strict understanding of 'normative'.
4. Normative Language off the (Standards) Track
This section examines the use of normative keywords in Informational
documents which are not protocol specifications. Some Informational
RFCs are in fact protocol specifications; this will be the subject of
Section 4.1.
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Despite the text of RFC2119, it is commonplace for normative keywords
to appear in Informational requirements documents today, in
statements that are intended to constrain the authoring of future
specifications. The laudable intent of requirements documents is of
course to establish consensus on the needs of the implementation
community prior to the evaluation of candidate protocol
specifications that might satisfy these needs. The requirements
document becomes a measuring stick of the 'compliance' of a candidate
protocol.
Undoubtedly some confusion arises from an accident of the language in
RFC2119. The Abstract of 2119 says that the normative keywords are
"are used to signify the requirements in the specification", which
could be read to suggest that Informational requirements that will be
used to constrain further protocol specifications should use
normative keywords. In fact, that interpretation clearly contradicts
the previously-cited dictum that normative keywords are to be used
only when required for "interoperation or to limit [implementation]
behavior."
Text at the end of Section 1 of RFC2418 furthermore suggests that
normative keywords might be applied by analogy to non-protocol
operations, in that case IETF process, in order to "reduce the chance
for confusion about the process", but it isn't clear how such an
analogy would operate. Were we to grant hypothetically that
normative keywords apply to requirements by analogy, the
interpretation of normative keywords in this context would remain
problematic. How are we to understand the "SHOULD" keyword for
protocol requirements, as opposed to protocols. What does it mean
for a protocol that satisfies a given set of protocol requirements to
be merely "conditionally compliant"?
Along these lines, it might seem compelling to imagine that the
selection of two protocols X and Y, which were invented to satisfy a
set of requirements A, might be decided by a single "SHOULD"
statement specified in A which is support by X but not Y. But of
course, if that "SHOULD" in A were instead a "MUST", the same
selection would be made. The true utility of a "SHOULD" emerges when
we instead consider two protocol implementations, X and Y, which have
been implemented to specification A and are attempting to
interoperate. In this case, if Y fails to implement a "MUST", a very
different result can occur than if Y fails to implement a "SHOULD".
In short, the normative keywords are designed to encourage
cooperation, not decide competition. Using them in the latter
context is a strained analogy, and the resulting strain rests on the
IETF's standards process.
It is moreover critical to appreciate that the use of normative
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keywords is tied to the functions of 2026: that is, the pruning of
unused features of a protocol specification. From the guidance in
4.1.2 (where we understand 'features' to be at the mandatory degree
of compliance, and 'options' to be at the recommended or optional
degrees of compliance):
The requirement for at least two independent and interoperable
implementations applies to all of the options and features of the
specification. In cases in which one or more options or features
have not been demonstrated in at least two interoperable
implementations, the specification may advance to the Draft Standard
level only if those options or features are removed.
Normative keywords exist to ensure interoperability; by contrast, a
requirements document will never be interoperable with anything.
More rarely, normative keywords appear in Informational frameworks
that describe high-level or abstract architectures. In this context
they are primarily used for rhetorical emphasis. This practice can
still lead authors of future specifications to improper referencing.
Finally, it is also possible for an Informational document to
redefine normative keywords in lieu of any reference to RFC2119.
This practice only adds further misery to the confusion surrounding
the use of normative keywords, and should be avoided. If there is a
genuine need for terminology to characterize adherence to a set of
requirements in the context of specification authoring, those terms
should be clearly defined and explicitly distinguished, semantically
and syntactically, from the RFC2119 normative keywords. A similar
direction should be taken regarding the use of normative keywords in
process statements. Further consideration is left as a possible
subject for future study.
4.1. Informational Publication of Protocols
There are a variety of circumstances in which protocol specifications
are published as Informational RFCs. Sometimes authors request
Informational publication of protocol specifications which were
rejected as candidates in a working group process in order to
preserve an historical record. Parties who do not participate
directly in the IETF may similarly request publication of their
designs as an Informational RFC. New ciphersuites designed outside
the IETF are typically documented, in strict procedural language,
within Informational RFCs as a convenient reference for protocol
designers; these latter are frequently a target of legitimate
downward references (see RFC3967). Some exceptional IETF procedures,
for example MIBs or the SIP change (RFC3427 [6]) process, may
stipulate a lower bar of review and Informational publication for
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certain protocol work.
These Informational documents often contain normative keywords, as
their authors aspire to specify something that will yield
interoperable implementations. One need not anticipate, or even
understand, the eventual intended status of a specification in order
to invoke RFC2119 and use the normative keywords therein. The
distinctions between such Informational documents and standards-track
documents lie more in the implications about the level of review and
community consensus which the standards-track entails than in any
consideration about the use of normative keywords.
Because such documents exist, it is not reasonable to bar
Informational specifications from containing RFC2119 normative
keywords. Indeed, the downref exception procedures of RFC3967 exist
so that it is possible to refer to such documents, under the proper
conditions and with the required oversight.
Instead, we should prevent the casual or inappropriate use of
normative keywords that to refer to matters other the proper
implementation of protocols.
5. IANA Considerations
This document contains no considerations for the IANA.
6. Security Considerations
This is a IETF process document which does not impact the security of
IETF protocols.
7. Acknowledgements
Harald Alvestrand, Brian Carpenter, Scott Bradner and Jonathan
Rosenberg have provided valuable input to this document.
8. Informational References
[1] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.
[2] Bradner, S., "The Internet Standards Process -- Revision 3",
RFC 2026, October 1996.
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[3] Bush, R. and T. Narten, "Clarifying when Standards Track
Documents may Refer Normatively to Documents at a Lower Level",
RFC 3967, December 2004.
[4] Klensin, J. and S. Hartman, "Handling Normative References to
Standards-Track Documents", RFC 4897, June 2007.
[5] Bradner, S., "IETF Working Group Guidelines and Procedures",
RFC 2418, September 1998.
[6] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B.
Rosen, "Change Process for the Session Initiation Protocol
(SIP)", RFC 3427, December 2002.
Author's Address
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
USA
Phone: +1 925/363-8720
Email: jon.peterson@neustar.biz
URI: http://www.neustar.biz/
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